The metal-semiconductor field-effect transistor is the product of a well developed and widely used technology. In its simplest form, a channel is formed in a surface region of a semiconductor, usually silicon. In a depletion-mode (normally on) FET, the channel is doped to be highly conductive. Source and drain electrodes are formed on opposite ends of the channel, and a metallic gate electrode is formed in the middle. The channel between the source and drain acts as the principal current path for majority semiconductor carriers. The junction between the metal gate and the semiconductor channel forms a Schottky barrier so that little current flows between the gate and the channel as long as the Schottky barrier remains reversed bias, for example, when the gate electrode is biased positively with respect to a p-type channel. Increasing reverse bias causes the depletion layer in the semiconductor to enlarge. Majority carriers are excluded from the depletion layer. If a sufficiently high reverse-bias voltage is applied to the gate to deplete the entire depth of the channel, the channel is pinched off so that the principal current path no longer conducts. If the channel is only partially depleted, the conductivity of the channel is proportionately reduced. Thereby, the gate voltage is used to either gate or modulate the source-drain current.
MESFETs and other FETs offer many advantages such as high-speed and low-power operation, but they suffer some disadvantages. Both p-channel and n-channel FETs are needed for complementary circuits, which dissipate very low power. However, the mobility of p-type channels is somewhat poor. In conventional MESFETs, pinch-off occurs as the depletion region meets the interface between high and low doping. Because such an interface is seldom abrupt, the electrical characteristics near pinch-off tend to be complex. Some attempts have been made to fabricate MESFETs with a dual gate, one above the channel and the other below so that pinch-off occurs in the middle of the channel. See, for example, "Novel Microwave GaAs Field-Effect Transistors," Electronic Letters, volume 15, 1979, pp. 627-629 by Vokes et al. However, such prior-art structures have been unduly cumbersome. It is also desirable to integrate a MESFET of a III-V compound semiconductor with a silicon integrated circuit.